1. Field of the Invention
The present invention relates to a semiconductor laser device for emitting a plurality of laser beams having different wavelengths and a method of manufacturing the same.
The present application claims priority from Japanese Patent Application No. 2002-374635, the disclosure of which is incorporated herein by reference in its entirety.
2. Description of the Related Art
In recent years, a semiconductor laser device called multi-wavelength laser for emitting a plurality of laser beams having different wavelengths has been under research and development.
For example, in the field of data recording and reproducing apparatuses with storage media called optical discs, typified by CDs (Compact Discs), DVDs (Digital Versatile Discs), etc., it has been of importance to develop a semiconductor laser device which can emit a plurality of laser beams having different wavelengths. The aim is to develop an optical pickup which has compatibility with various CDs, DVDs, and the like commercialized heretofore and can cope with new storage media capable of recording at higher densities.
For such a semiconductor laser device, ones having a hybrid structure have been proposed (for example, Japanese Patent Laid-Open Publication No. 2001-230502: hereinafter, referred to as patent document 1) in order to solve difficulties in implementing it as a monolithic semiconductor device.
As disclosed in FIG. 1 of the foregoing patent document 1, the semiconductor laser device comprises a first light-emitting element and a second light-emitting element which are manufactured on chips separately. The first light-emitting element has a GaN-based laser part which is formed on a first substrate and emits a laser beam of short wavelength (for example, in a 400-nm waveband). The second light-emitting element has an AlGaInP-based laser part and an AlGaAs-based laser part which are formed in parallel on a second substrate and emit laser beams of longer wavelengths (for example, in 600- to 700-nm wavebands). These first and second light-emitting elements on chips are stacked and mounted on a support substrate (so-called submount) to achieve the semiconductor laser device of hybrid structure.
Here, the first light-emitting element is mounted on the support substrate with its GaN-based laser part between the first substrate and the support substrate. The second light-emitting element is mounted-on the first light-emitting element via the first substrate with its AlGaInP-based and AlGaAs-based laser parts interposed between the first substrate and the second substrate.
Consequently, in this structure, the GaN-based laser part for emitting a laser beam of short wavelength (for example, in a 400-nm waveband) is attached to the support substrate, the AlGaInP-based and AlGaAs-based laser parts for emitting laser beams of longer wavelengths (for example, in 600- to 700-nm wavebands) are attached to the GaN-based laser part via the first substrate, and the second substrate is located on these AlGaInP-based and AlGaAs-based laser parts.
This semiconductor laser devices is then mounted on an optical pickup. The GaN-based laser part emits the laser beam of short wavelength to perform information recording or information reproduction on a storage medium capable of high-density recording. The AlGaInP-based and AlGaAs-based laser parts emit the laser beams of longer wavelengths to perform information recording or information reproduction on various CDs, DVDs, and the like commercialized heretofore. Thus, a compatible optical system is realized.
By the way, the conventional semiconductor laser devices, as described above, have the structure that the first light-emitting element and the second light-emitting element are manufactured as separate semiconductor chips in advance and the first and second light-emitting elements on chips are stacked and mounted on the support substrate (submount). The light-emitting elements on chips must therefore be aligned and assembled with extremely high precision so that the light-emitting elements emit the respective laser beams from their cleavages in the same direction. This means a problem of extremely complicated manufacturing steps, for example, in manufacturing semiconductor laser devices intended for an optical pickup.
In the conventional semiconductor laser devices, the GaN-based laser part of the first light-emitting element is mounted close to the support substrate while the AlGaInP-based and AlGaAs-based laser parts of the second light-emitting element are mounted on the first substrate which the first light-emitting element is provided with.
According to this structure, however, the first substrate which has a large thickness lies between the first and second light-emitting elements. As also discussed in the foregoing patent document 1, the first substrate (GaN substrate) typically has a thickness of the order of 100 μm. This produces the problem that the position of emission of the laser beam from the GaN-based laser part (the position of the light-emitting spot) and the positions of emission of the laser beams from the AlGaInP-based and AlGaAs-based laser parts (the positions of the light-emitting spots) are wide apart. In short, there occurs the problem that the distance between the light-emitting spots of the laser beams increases.
For example, take the case where an optical pickup incorporates this semiconductor laser device for information recording or information reproduction. If the position of emission of the GaN-based laser part (the position of the light-emitting spot) is centered to the optical axis of the optical system constituting the optical pickup, then the direction of the laser beams emitted from the AlGaInP-based and AlGaAs-based laser parts may deviate greatly from the optical axis of the optical system owing to the thickness of the first substrate. This can sometimes cause aberrations and the like.
In another case, a prism or any other optical element may be added to eliminate the adverse effect of the thickness of the first substrate, for example, in order that the laser beam emitted from the GaN-based laser part and the laser beams emitted from the AlGaInP-based and AlGaAs-based laser parts are all centered to the optical axis of the optical system in the optical pickup. This, however, produces such problems as an increased parts count.